Stimulated Brillouin Scattering (SBS) refers to nonlinear effects in high power fibers under which optical power is unintentionally scattered inelastically in the backward direction. Scatter in the backward direction is caused by an interaction of photons and acoustic or vibrational phonons. Acoustics in the fiber are caused by electrostriction due to the electric field produced by the beam of light in the fiber. Acoustic energy in the fiber causes light to be reflected in the backward direction and consequently interferes with the propagation of light forward. This results in a drop of useful output signal. The frequency of the back scattered beam is slightly lower than that of the original beam.
The frequency shift corresponds to the frequency of emitted phonons due to the Stokes process. This shift, known as the Brillouin shift, is equal to the energy of the phonon excited by the propagating laser signal. The Brillouin shift is commonly measured by the use of an optical spectrum analyzer, or is based on a Fabry-Perot interferometer for high resolution measurements. At a sufficiently high power in the forward direction of the fiber, some of the signal is back scattered at the Brillouin shifted frequency. SBS is a process whereby nonlinear optical gain produces amplification of the back scattered light. That is, the interference with the light being propagated in the forward direction is not linearly related to the optical power. Instead, SBS effects are not incurred until an SBS threshold of optical power is met. Soon thereafter the SBS back scattered optical power overwhelms the optical power in the forward direction. Therefore, the higher the SBS threshold the more power can be transmitted successfully through the optical system.